Television echo suppression system



Feb. 21, 1950 F. J. BINGLEY ET AL 2,498,391

TELEVISION ECHO SUPPRESSION SYSTEM Filed March 22, 1945 7 Sheets-Sheet l FRAME FRAME FRAME NQI No.2 NO-3 FIG. 3. FIG. 4.

INVENTORS FRANK J. BINGLEY BY NEH/LAND r'. 5M/TH,./R.

("PM Mac 14/40 ATTORNEYS Feb. 21, 1950 F. J. BINGLEY ET AL 2,498,391

TELEVISION ECHO SUPPRESSION SYSTEM Filed March 22, I945 7 Sheets-Sheet 2 I I I FIG.5 I

o i o FIELD 2| FIELD 22 I I I I I I I I II I II FIELD 2| FIELD 22 FIG6 I w I a II II I II II FIELD 3| FIELD 32 I I, I

I I I II I II II I I I I FIG7 I I I I I I I I INVENTORS FRANK J BIA/GL5) BY NEW/.A/VD F. 541/ TH, JR,

ATTORNEYS Feb. 21, 1950 F. J. BINGLEY ET AL TELEVISION ECHO SUPPRESSION SYSTEM 7 Sheets-Sheet 3 Filed March 22, 1945 Feb. 21, 1950 F. J. BINGLEY ET AL 2,498,391

. TELEVISION ECHO SUPPRESSION SYSTEM Filed March 22, 1945 7 Sheets-Sheet 4 4 r M I m @MW 4 5 m m 3 mm; w W \NEG E WWW QM m BK IMM. A WM kfinfis m8 mqfifis E933 m2 WW +2 v w y mwwmq WW a om wk b3 &. 33 1 RE mwmfin 03+ b J mm AV l m P 2 r MEG E m p u 2m w W 1| W r ||||l|. IL 9 3 @fiww E Q 7 Shecs-Sheet 5 kmimk QR w wwlvih n m I mo T J [IIL N Mbmo INVENTORS FRANK J. @I'NGLE NEH/LAND F. summ ATTORNEYS Feb. 21, 1950 F. J. BINGLEY ET AL TELEVISION ECHO SUPPRESSION SYSTEM Filed March 22, 1945 Feb. 21, 1950 F. J. BINGLEY ET AL TELEVISION ECHO SUPPRESSION SYSTEM '7 Sheets-Sheet 6 Filed March 22, 1945 Hninuu Patented Feb. 21, 1950 TELEVISION ECHO SUPPRESSION SYSTEM Frank J. Bingley, Philadelphia, andNewland F. Smith, Jr., Abington, Pa., assignors, by mesne assignments, to Philco Corporation, Philadelphia, Pa., a corporation of Pennsylvania Application March 22, 1945, Serial No. 584,112

Claims. (Cl. 178'7.2)

This invention relates to picture transmission systems and the like, and to a method and means for controlling the action of the residual echoes after suitable provision has been made for the elimination of the major part of the echo effect upon the received or reconstituted picture.

More particularly, the invention relates to television systems and to a method of controlling the behavior of the residual pattern which remains after certain means for the rejection of echo signals have been employed. In particular, it is applied to a method of eliminating the undesirable progression of echo residual patterns resulting from a double reception of the synchronizing pulses caused by the fact'that radio waves travel over two paths from the transmitter to the receiver when a reflecting structure is near the receiver.

One of the problems which occurs in television transmission is that which arises from long delayed echoes which occur when radio waves arrive at the receiver over two different paths; one path of which is the direct path and the other path ofwhich is a path involving reflection from an object, as for example a cliff or a gas holder, which is off to one side of the direct path b a considerable distance. Such multiple paths may produce echoes delayed by or microseconds. In magnitude'these echoes may be relatively faint, but since the blanking and particularly the synchronizing signals are of considerably greater amplitude than the line or picture portion of the television signals, echoes of the reflected blanking and synchronizing signals are sometimes of considerable strength and appear in the picture as a result of these long delayed echoes.

The forms which such echoes may take are clearly shown in a pending application of Bingley and Bradley, Serial No. 433,660, filed March 6, 1942, now Patent No. 2,386,087, issued October 2, 1945. In that specification, it is shown that an echo signal causes either a dark or a light spot upon the picture as received, and also that a series of vertical lines alternately dark and light are obtained when echoes of the synchronizing pulses are received using the system of alternate carrier frequency for the synchronizing pulses as described in Patent No. 2,347,248.

In the co-pending application referred to, it is shown that when the effect of an acho is normally black, it can be changed to white by reversing, in an alternate sequence, the polarity of the carrier of the synchronizing and blanking signals. This polarity reversing may be done in any one of a number of ways, but it appears to be preferable to reverse these polarities for every successive synchronizing pulse throughout one field. This produces echo signals which if examined from the top to the bottom of the picture screen appear as a series of dark and light spots.

In order to produce neutralizing of this echo effect, it has been proposed that the corresponding field in the succeeding frame should have the polarities of the echo signals just reversed from those which obtained in the first frame. Since these frames occur at a rate of 30 per second, it was expected that the eye would average out the alternate dark and light spots so as to neutralize the echo effect. Thus the eye seeing alternately light and dark spots would average the light intensity, and the screen at the position of the spot would appear to'have the light intensity that was supposed to be present without the echo, so that apparently there would be no echo in the picture.

It was found, however, that with this type of switching of the phases, there was an apparent vertical optical progression across the screen of a series of dots. Under some circumstances this series of dots appears to move upward, and under other circumstances, this series of dots appears to move downward. This effect, while considerably less objectionable than the original echo effect which it was desired to overcome was nevertheless somewhat troublesome, and it is the purpose of this invention to control the motion of these dots.

In accordance with our invention the actual reversal of polarity is accomplished by circuits, such as those described in the application referred to above. The particular features of this invention are the means by which keying signals are supplied to these polarity reversing circuits in order to obtain certain specified sequences of polarity in the synchronizing pulses transmitted by the television transmitter.

Accordingly a primary object of this invention is to provide means for obtaining a certain sequence or sequences of polarity reversals of the synchronizing pulses in a television transmitter.

Other objects and advantages of the invention will be apparent from the following description and the accompanying drawings in which:

Figure 1 shows two successive frames of the television screen with the received echoes shown symbolically.

Figure 2 shows a diagrammatic development of the echo images shown in Figure 1.

Figure 3 shows a symbolic development of ansequence of polarity reversals.

Figure 4 shows still another method of echo progression.

Figure 5 shows a video signal with synchronizing pulses.

Figure fi shows two successions of pulses for producing the patterns of Figures '2 and 3 re spectively.

Figure 7 shows two series of pulses used to form the series of Figure 6.

Figure 8 shows a block diagram of a system tor producing the synchronizing pulses required in Figure 6a to produce Figure 3.

Figures 9 and 10 show a schematic diagram developing the details of the blocks shown in Figure 8.

Figure 11 is a schematic of a carrier phase keyer.

Figure 12 shows the relative impulses on th various lines and their time relation. "The present invention can be best understood by first referring to Figure 1. In this figure there is shown two successive frames of the picture on the television screen. This picture has been simplified in two respects. Fir-st of all, the actual television picture itself has been eliminated from the diagram, the normal screen appearing merely as a shaded or dotted region. Another simplification has been made in that the number of scanning lines has been reduced to 9 instead of the customary '525. This has been done merely for the purpose of making the description of the invention more clear, since the same theory which will be described hereafter will hold for any number of scanning lines arranged as this sequence'of scanning lines is arranged.

In scanning each frame, there are two fields, the first of these constitutes scanning lines I, 2, 3, Aand 5. The second field constitutes lines "6, 1, "8 and 9. It will be observed that the first sequence of scanning lines is interlaced between the second sequence of scanning lines. In the frame numbered 2, the same sequence of scanning line's is carried out, the lines now "being numbered from I! to l9. Theeffect of the echo of the synchronizing and blanking pulses are shown in these figures. In onephase the echo is shown to give a black signal; in the opposite phase it is shown as giving a white signal. These, of course, may be in any difference of gradation since in vector terms the phases, although they are diametrically opposed 180, may be at any random angle to the actual picture signal ca rier. In the diagram the worst case has been shown, in which the echosignal isfirst fully. in phase with the picture signal and then fully out of phase with the picture signal. As the picture is scanned on scanning line #1, the echo givesa response of one polarity. On scanning line #2, the echo gives aresponse of the other polarity, since the phase of the echo is now the opposite tothe 'phase which existed when 'line 'I was be- 'ing scanned. Likewise, in scanning line 3, the polarity of the echo has been reversed to the polarity given with line I. This process is, continued through the end of the first field.

' In the system shown in this figure, the same sequence is continued to line 6, line '6 giving a white echo when line 5 gave a black echo. This alternate sequence 'is continued down to line 9, and line H, which follows "line 9, gives 'a'white signal in contrast to a 'blacksignal'trom'l-i-ne 9.

This sequence of echo polarity is shown in Figure 2. Here the vertical column of squares above #Zl is the column of echoes produced in the scanning of the first field of frame #I. The vertical column of squares above 22 is the pattern of echoes produced in scanning the second field of frame #1 and so "in sequence across the diagram from left to right.

Thus in scanning field 2|, the polarity of the echoes could be described as being positive, negative, positive, negative, positive, reading from top to bottom of the column-that is, from top to bottom of the screen itself. It is to be observed that every other square is left blank because the scanning process for the first field of frame #I omits every other scanning position.

Field #2 of .frame #I fills in these omitted scanningpositions, and thus it is observed that the polarity marks in the second field of frame it I, that is, field 22, do not appear in the same horizontal position that do the polarity marks for field 21. Field 23' occupies the same scan- "ning position as does field 2| and likewise does field 25. Moreover field 2'4! and field 2B occupy the same scanningposition that field 22occupies.

If then, the polarit of the'eohoes is-consistently reversed, minus, plus, minus, plus, minus, etc. progressively from-field to field, a pattern is secured of plus andminusmarks in the polarity diagram for the echoes corresponding to the arrangement shown in Figure 2. The special feature of this form of diagram arises from the fact that it is now easyto see whether or not there is any. apparent motion of the'echo pattern -upon'the screen. This part-icular'diagram shows that the apparent motion of the echo pattern will be upwards. This is shown by the fact as one moves his vision from left to right across the diagram, the polarity marks rise through the diagram. This then gives an eifect upon the television screen of an upward motion of the bright and dark spots o'f'the echo pattern.

Psychologically, it appears that this upward motion of the echo pattern is more objectionable than would be a downward motion-of the echo pattern. Such a downward motion "would appear in an echo sequence diagram as is shown in Figure 3. Here it is'observedthat the polarity marks progress downward as one goes from field to field across from "left to right so the apparent motion of the echoes is downward. Instud ying the sequence .of these echo polarities, it isobserved-that although through each field the echo polarities automatically reverse from plus to minus to plus to minus, 'etc., when the transition ismade from one field to the next field, the polarity is *maintained the same at the beginning of the second field as it was atthe end of the first field. Thus the first polarity. mark, that is, the top one in 'field .32 is positive, and likewise t-he last polarity marks in field "31 that is, the bottom one, is also positive. Thus as one goes from field 3 I to field 32 one must also reverse the reversal of polarity marks. Itqis to be-observed that this reversal of reversals again occurs between frame 32 and 3:3; likewise between frame33 and 34 and soon across the whole picture pattern.-

"It is a purpose of the invention to show a method for accomplishing this "type of reversal. Itis, of course, possible byvarious combinations of the pattern of Figure 2 and-the pattern of Figure 3, to accomplish a succession :of motions up and down of the echo pattern on the screen. Such a "succession is shown in Figure =4' i-n which there are four downward motions, then four upward motions, then four downward motions, etc.

The manner by which the synchronizing pulses change polarity in the manner shown in Figures 2 and 3 can also be demonstrated with the aid of Figure 5. This is intended to correlate with the diagram of Figure 2, the practice preceding the present invention, and shows the end of field 2I and the beginning of field 22. Here the lowest or last positive polarity in field 2| is shown and the first or negative polarity in field 22 is also shown. The upper part of this diagram of Figure 5 shows the variation in transmitted signals showing the blanking pulses, the synchronizing signals, and the video parts of the signal. In this diagram time is represented as moving from left to right. The polarity diagram of Figure 6b beneath this transmitted signal diagram indicated is so arranged that it indicates positive and negative polarity of the transmitted carrier signal.

In accordance with the previously mentioned application, the polarity is changed during the blanking and synchronizing pulses for every other pulse giving them a positive polarity for every other synchronizing pulse and a negative polarity for the remaining intermediate synchronizing pulses. This provides the echo pattern as is shown in Figures 1 and 2.

In order to achieve the sequence of echoes as shown in Figure 3, it becomes necessary to have a polarity sequence such as shown in Figure 6a. This polarity sequence is in time register with the video signals shown in Figure 5. It will be observed from Figure 3 that in changing from field 3| to field 32 there are two successive positive polarities involved.

It is a necessary part of this invention that. arrangements be made for accomplishing this purpose. One such arrangement can be obtained through the use of a pair of polarity signals, such as are shown in Figures 7a to 7b. It will be recognized that 711 is the same signal used in Figure 612 for polarity reversals, and that the signal in Figure 7b is complementary to that shown in 7a. The keying signals shown in Figure 6a can be obtained from the signals a and b of Figure 7 by using signal a for field 31 and signal b for field 32. This then requires switching from signal a to signal b at the time that the fields change from 3| to 32. Likewise, at the time that field 32 changes to field 33, it is necessary that the keying'signals be switched back to signal a. At the time that field 33 changes to field 34, the switch must be made again so that polarity sequence 5 is used. This change, it will now be noted, occurs every time that one field changes into the next field.

A block diagram for a system of accomplishing the desired result is shown in Figure 8. Horizontal blanking pulses at 15.75 kc. are fed in over line 8| and through a buffer amplifier I01 and line I06 to a differentiator circuit 82. This differentiator circuit is so arranged that its output, which provides a sharp pulse occurring at the lagging edge of the input pulse coming over line 8I, is fed over line 83 to the delay circuit. This pulse is inserted into the delay circuit 84 and the output of the delay circuit on line 85a is a pulse delayed slightly from the pulse which comes in on line 83. Thus the pulse on line 85a is delayed after the pulse entering on line 8|. The pulse on line 85a is fed into a multivibrator operating at approximately a frequency of 7.875 kc. which is half the frequency of the input pulse system.

The signal on line 85 synchronizes the multivibrator so it is maintained atexactly half of the frequency of the input pulses. The output of the multivibrator 88 is fed over line 81 to a phase inverter 88. This phase inverter produces two series of square wavesphase I on line 89 and phase 2 on line 90. These two phases are in opposition to each other, and are fed into individual gate circuits I93 and 99, respectively.

The vertical synchronizing signal is brought in over line 9| and constitutes pulses at 60 cycles, each pulse lasting for 9 horizontal line periods in the illustration here shown. This input signal is fedinto a multivibrator 92 which operates at cycles per second. The output of this multivibrator on line 93 is in the form of approximately square waves at 30 cycles. This output is fed into the shaper and phase inverter 94. Out of this shaper and phase inverter two phases of 30 cycle square waves are obtained-one on line 95 and another on line 96.

The phase A on line '95 is suppliedto gate #I and when the signal present on line 95 is of the proper polarity, gate #I is operative and phase'I is put out of gate #I through line 91 into the adding circuit 98. Whenthe voltage on line 95 is of the oppositepolarity, the voltage on line 96 is proper to open gate #2 indicated at 99, so that the signal online 99 is now presented to the adding circuit 98 through line I80.

The adding circuit 98 takes the output of gate #I and gate #2, adds them, and feeds them to gate II. This gate is controlled from the 60 cycle 9 line pulses so that this horizontal signal is turned off during the time that the vertical synchronizing signal is being applied. The output of gate IN is fed over line I02 through the shaper amplifier I to the transmitter.

This output then consists of a series of pulses occurring a double line apart, except during the vertical synchronizing pulses when they are blanked out. These are shifted in timing by a single horizontal line at each change of field. This is the signal illustrated in Figure 6a.

If it is desired to operate this system without shifting the pulse sequence each field, this can be done by biasing off gate #2 the phase inverter 88 and the phase inverter 94 in such a manner that lines 89 and line 95 are continuously energized. Under these circumstances gate #2 is not operative so the signal coming through gate IOI remains always the same. That is, a succession of pulses occurring at a frequency of 7.875 kc. Each one of these pulses then would be used to shift to invert the phase of the carrier during the time of transmission of the horizontal blanking pulse.

In the gate IOI the complete sequence of 15.75 kc. pulses is controlled by the output of the adding circuit 98. Thus the output of the gate I82 is composed of the sequence of pulses desired, as is shown in Figure 6. These signals are actually part of the signals injected into the input of the system and thus have no phase shift in them.

In Figure 12 I have illustrated the relative time relations of the signals. Referring to this figure and summarizing the operation of the keying signal generator, the signals on line 8| of Figure 8 consist of a series of pulses at a frequency of 15.75 kc. as shown in this sheet of wave forms at a. These pulses are sent through the buffer amplifler and differentiator and delay circuit of Figure 8. The output of the differentiator consists of a series of pulses occurring at the beginning and end of the input pulses. Only the pulses at the end are used in the delay circuit.

Thesepul'sesxgenerateia series of transientsfin the output of. the. delay :circuit. These are. shown as lineb .in Figure 12. and occuratthe same free quency:of.15.75 kc. That is, this isthe repetition frequency of these. transients- It is to be observed that. these transients are occurring in .the "time interval provided for the signal shown=in a. for thevideo signals. These transients: are used to synchronize'the 7.875 kc. multivibrator; The output of thisrmultivibrator is substantially. a square .wave outputawithafreqency of 7.87.5'kc.v ,Its wave shape appears in line of Figure-.12. This signal .ispassed through the phase inverter ':Y which provides :two different signalsas output signals. One of thesenis just like the phase inverter inputsignal' and :appears on line. 90in Figure 8 "and its wave form is shown as c in Figure .'12.;.The other output signalof this phasednverter appears, on. line 89 and; is shown as d in Figure 12.

It is to :be observedthat'the vertical-portionof these square waves occur'during the quiescent period of the signal input shown: aswa. Thus, there is nopossibility that the subsequent operations caused by thesetwo square waves-c and d will interfere with theoriginal pulses put into the system.

The 30 cycle multivibrator'fizof Figures generates square wavesvat: a frequency of 30 cycles. The output of this' multivibrator is likewise fed into a shaper and phase inverter 94,the output of which-appears as two signals, one on line 95 andone 'on'line 96.

In Figure '12' it is only possible to show the change-over period of thissignal-since the period is so-long. The signal shown as e iii-Figure .12 is the signal'on line 95 and the signal shown as j is the signal on line 96-. The signal e is-used in gate #I tokey insignald, thus providing the signal shown as g. This signal appears on line 97 The operation ofgate #I then is' essentially as follows: when the signal on line 95 is positive, gate #I is closed, and there is no output. When the signal on line 95 drops to-a lower value; gate #I 'opens'and the-signal on lineBBis'transmitted to line 97. i

A similar action occurs in gate #2 in which the signal on line 96;.that is,-the signal I in Figure 12, control s'thefpassage w nc -passage of the I signal on line 90. shown as c in-l' igure 12.

The. resultant output of. gate .'#2 appearing on line I00 is then waveshape h in Figure 12. It is tobe. observed that wave shape it containsthe 7.875 kc. .multivibrator output until the change- This signal.i-is'the-.signalwhich.controlsthe gate- I B I. When .this signal voltage. is. negative,

the. gate open and. -when thissignal is; positive,

the gate closed... This agate-operates on the original-15.75 kc. pulsei-series coming .over line lli land serves. to admit pulsesinaccordance with theresultant. keying-signal Thusthe output of: this. gate. onz-line-IM- consists .of a series .of pulses {shown-as linej in Figure 12 which corresponds to; :the, desired .series of pulses. shown as 8; in Figurea6a,"-- except. for theipolaritygwhich is inverted-in the shaper amplifier I05. 7

The mannerinwhichthis signal is obtained can be seenby correlating wave shape 2' with wave shape-a. When-i isnegativathe pulse in-a appears in 7'. Whenz' is positive, the pulse in a does not appear. in becausethe. gate IOI is closed. Since 2' has a 7.875 kc. frequency, it blanks out everyrother pulsei'in the wave shape a.

A circuit. diagram for accomplishing these operations is.-shown.in Figures 9 and 10. .The parts of this diagram which correspond to the blocks on.the:block diagram are indicated by dotted lines which enclose parts of the actual circuit diagram. These lines are labeled with the .same numbers that occurred in Figure 8 showing the relationship between the blocks. in Figure '8 and the actual circuit diagram.

This circuit takes the 15.75 kc. horizontal blanking pulses in tube I I I, further amplifies this initube H2, and-bydifierentiating the. pulses a wave front is derived'from'the trailing edge of the. pulse which is-used to impulse the. transient delay circuit inthe plate of tube H3. The 7.875 kc. multivibrator, tube .I I5, istherefore fired. by this delayed wave form. This 7.875 kc. pulse is shaped up in tube I2 I1 and two polaritiesobtained, one from the plate circuit and one from the.cath ode circuit, called on-the diagram I and.2.- A cycle waveform is generated in:the multivibrator, tube I09, by dividing down the 60. cycle pulse input from-the synchronizinggenerator. This 30 cycle square wave is applied through tubes H0, H1, II8-and I I9 to alternately switch on and off tubes I22.:and. I23... This. is accom plished by the common cathode resistors between tubes I28. and I23, andalso between .tubes H9 and I22; Tubes'l22and I23 have impressed upon their grids phase one (I) andphase two (c2), respectively, of :the 7.875 kc. pulses. In the common plate load of tubes I22 and I23, there is, therefore, for one frame this seriesofphase one (cl) 7 .875 kc. pulses; forthe following frame the series of phase two; (2) 7 .875 kc. pulses. These 7.875 kc. pulses are then. amplified andshaped up in tube I24 and applied to .the grid of tube I25. Tube I25 is usedto turn tube IZIi off and chat a 7.875 .kc.-.rate. Thegrid of tube I2! is fed with the regular series of 15.75. kc..blanking pulses through theamplifier,andinverter I26.

Theoutput of tube 121 therefore contains a series of blanking pulses with everyother one being omitted, :the sequence of these being shifted every .field 0/60 of .a second)- Byopening the switches inthe gridcircuits of tubes III] and I23 there could be produced-a normalsucces'sion of keying pulses at 7.875 kc. without the sequence beingshifted every field. Thisnormalsuccession'of pulses produces; the raining..up effect shown in Figure .2. Shifting the sequence every fieldwiththe 30. cycle..wave-.form.produces the rainin down effect shown .inlFigure 3.

The keying operations are withheld. for .a period .of time aroundthe vertical synchronizing signal. Thisisdoneintube I [6 by applying the -cycle nine-line? pulsetothe' grid of tube H6 which in turnturns off tube IZI forthis period and prevents: any keying .pulses from appearing in the. output.-

Thetoutput of-Figure 10 is in the form. of a series of positive .keying pulses.

In order to turnthese. polarities into actual changes in: phase'of the-carrier frequency, a circuitsuch-as, thatof Figure .11 may be employed. Thecarrierfrequency is fed .into the radio frequency transformer I50. The output of this transformer is .in two phases, since the center tap of the secondary of the transformer is grounded. Thus the phase on line II is 180 out of phase with the signal on line I52. This transformer then acts as a source of two different phases of carrier frequencies, these two phases being 180 out of phase with respect to each other. These two signals are fed into the grids of tubes I53 and I54.

The cathodes of these tubes are by-passed to ground for carrier frequency by their respective condensers I55 and I56. Condenser I55 is shunted by resistor I51 and likewise condenser I56 is shunted by resistor I58. These resistors also form the cathode resistors of a pair of keying tubes I59 and I60.

These keying tubes are fed from a phase inverter amplifier tube I BI which receives its input from the keying signal generator which has been described in connection with Figure 8 and specifically from the keying signal generator of Figures 9 and 10.

Consider now that the input to the tube I6I is low in potential-that is, in the waiting period between pulses. Under these circumstances, tube I6I is non-conducting. Consequently the grid of tube I59 is high in potential and the grid of I60 is low in potential. Since tube I59 conducts, a large bias voltage is developed across resistor I51. This large bias voltage effectively cuts oil tube I53 which then acts as an open circuit. Tube I50, however, is cut off by the low potential on its grid. Consequently the only current through bias resistor I58 comes from tube I54 and this current develops the correct amount of bias voltage to cause tube I54 to act as an amplifying tube.

The input to this tube is the carrier frequency signal on line I52. The output from this tube goes to the radio frequency transformer I62 and is then fed to the buffer amplifier and finally to the modulated amplifier in the transmitting systems. This modulated amplifier is the amplifier which receives the video modulation signal which is to be transmitted by the television transmitter.

Now consider what happens when a positive pulse from the keying signal generator comes into tube I6I. Under this circumstance, this tube will conduct and consequently the grid of tube I59 falls in potential and the grid of I60 rises in potential. These changes are of a sufiicient magnitude to cause tube I59 to become cutoff and to cause tube I 60 to conduct. Conduction in tube I60 sets up enough voltage across resistor I58 to cut off tube I54. In a similar manner lack of conduction in tube I59 causes a relatively small bias to be generated across resistor I51. This is just the bias caused by cathode current in tube I53 which then acts as an amplifying tube. Its input is the signal on line I5I, and its output is again given to radio frequency transformer I62 and is passed on to the bufier amplifier and finally to the amplifier which is modulated with the video signal to be transmitted.

Thus it happens that during the time that a keying signal is received on tube I6I, the carrier frequency transmitted to the modulated amplifier has its source as the phase of carrier frequency on line I5I, and during all other times, the carrier frequency has its source at the opposite phase, namely that on line I52. Consequently, whenever a keying signal is received by tube IGI, the signal to the buffer amplifier and Similar block diagrams could be drawn to accomplish the sequence shown in Figure 4.

Such diagram would, of course, contain more parts than the diagram of Figure 8 because the sequence is more complicated. necessary to provide first the signal of Figure 6a continuously for a number of fields, and then to reverse the situation so that the signal of Figure 6b was applied for an equal number of fields. Then the reversal would have to be made back to the situation givingthe signal of Figure Go for the same number of fields. Such an arrangement could be made with an additional electronic switch coupled into the circuit ofFigure 8 and controlled by a frequency dividing circuit which would operate from the vertical sweep synchronizing signal source of Figure -8. 3

While for simplicity, the system has been described in terms of an amplitude modulation system, it is equally applicable to the alternate carrier system shown, for example, in Patent No;

Many such modified systems could be arranged and consequently we prefer that the invention not be limited by the descriptions of specific ein-I but bythe bodiments which we have given here, following claims.

We claim: 1. In a television system, a source of carrier current, a source of horizontal synchronizing pulses, a differentiator circuit, a buffer amplifier for applying said synchronizing pulses to said differentiator circuit, a delay circuit connected to said differentiator circuit for providing sharp pulses occurring at the lagging edge of said synchronizing pulses, a multi-vibrator operating at half the frequency of said synchronizing pulses activated by said delayed sharp pulses, a phase inverter connected to said multi-vibrator for producing two series of square waves in opposition to each other, a pair of gate circuits individually energized by said series of square waves for sequentially opening said gate circuits, a source of vertical synchronizing pulses, a. multivibrator energized by said vertical synchronizing pulses for producing square waves in accordance with the frequency of said square waves, a phase inverter for generating signals of two phases from said square waves having a frequency in accordance with the vertical synchronizing pulses, means for supplying said signals of two phases of the vertical synchronizing pulses individually to each of said gate circuits, the first of said phases being fed therethrough when its associated gate is operative and the other of said phases being fed through its associated gates when its gate is operative, and means for controlling the phase of said carrier current in accordance with the signals fed from said gates.

22. In a television system, a source of carrier current, a source of horizontal synchronizing signals, means for generating a pair of signals out of phase with-each other from said source of horizontal signals, means including cir- It would be.

Quit connections, for" controlling the; phase J of said 'carrier currents in: accord-ance withsaid pair; of phase displaced signals, said means comprising a-pair of gates, each of said gates-being individual to said phase displaced signals, means iorrenderingsaid gates successively operative, said-last means-comprising a source of vertical synchronizing signals, means for generating two signalsout oi phase with respect to each: other from sa-id vertical synchronizing signals, and means'including circuit connections to said gates for individually applying said two last mentioned qut 'of phase signals to said gatesto render said gates operative. 1

a a television system, a" source of carrier current,-- a source- 'of horizontal synchronizing signals means for generating apair of signals 180? out of phase witheach other fromsaid source-of :horizontal signals, -means including circnlticonnectionsz' for controlling the phase of said carrier. currents in accordance with, said 'of phase displaced signals, said means comprising a. pair oj gates; each of saidqgates being individual to said .phaser displaced signals, means forurendering said-zgg-ates: successively operative, and switching-rmeans f!" controlling the operatiom pfiesaidrlasts mentioned means 1on1 gates so as to changedme sequence of --oper-ation of said gates.

4nIn' artelevision; -system,-. a source of carrier current',.i a;-, source r, of a horizontal.- synchronizing $ina1s,.;- means for generating. a: pair of signals 180 out of phase with each other from :said source of; horizontal ,signals, .:m.e.ans including circuitecennections ior controlling the phase. of s carrier currents;- .my.acc rdan ithasaid Danni phase lisnlaceds nals, saidlmeans, comprising a pair of gates, each of said, gates being individual to said phase displaced signals. means $01,: rendering, said gates successively operative, saidllastmeans comprising a source of vertical synchronizing signals, andv switchingmeans for controlling the operation of said last mentioned means on gatesso as to change the sequencev of operationot said gates.

5;- -In a-televisiorr system a: source-oi carrier current,- asourceof horizontal synchronizing signals, means 'for generating a 'pair of signals 180 out of-phase with :each other' from said source of horizontal-signals; means including circuit connections for controlling the phase of said carrier: currents :in accordance with said pair of phase? displaced signals; said means :comprising a'pair of. gates, each of :said gatesbeing individual to said phase displaced signalsg: means for: rendering. said:gates/successively operative, saidula'st means com-prising. a source ;of vertical synchronizing signals,;means for generating two signals out. of. phasewitlr respect to each other iromzsaid vertical synchronizing signals; means including-circuit :ccnnectionswtomsaid gates :for individually applying said ltworflast: mentioned out ofphase lsignalstowsaidcgates rto'renderis'aid gatesr operative; andswitchin means for controlling, jthex operation of; said :last :mentioned means :ongates so as to changefthe;sequence of operation :ot said. gates.

' The followingsreferencesa arez oi. record in' the file 'ofrthis' patent:

JUNITED "STATES PATENTS.

' OTHER; REFERENCES Television -Standards. and Practice, by Donald G. I Fink, =McGrawv-Hill, 1943, pages 253-! 260. 

